1. Field of the Invention
The invention relates generally to the production of a wood composite having superior surface quality. The invention also relates to a process for the production of a wood composite which overcomes the disadvantages of known solution treatments. More particularly, the invention relates to the minimization of the amount and integrity of undesirable materials which tend to build up on die surfaces utilized during consolidation of the wood composite.
2. Brief Description of Related Technology
Wood composites, e.g. hardboard or fiberboard, may be formed in desired shapes and sizes depending on the intended use, for example as a doorfacing or doorskin which is applied to a door body. The principal processes for the manufacture of wood composites include (a) wet felted/wet pressed or "wet" processes, (b) dry felted/dry pressed or "dry" processes, and (c) wet felted/dry pressed or "wet-dry" processes.
Generally in a wet process, cellulosic fillers or fibers (e.g. woody material which is subjected to fiberization to form wood fibers) are blended in a vessel with large amounts of water to form a slurry. The slurry is deposited along with a synthetic resin binder, such as a phenol-formaldehyde resin, onto a water-pervious support member, such as a fine screen or a Fourdrinier wire, where much of the water is removed to leave a wet mat of cellulosic material having, for example, a moisture content of about fifty weight percent. Where two separate water removal steps are utilized, these steps may be referred to as the primary water removal step and the secondary water removal step. The wet mat is transferred from the pervious support member to a press and consolidated under heat and pressure to form the molded wood composite.
In a dry process, the cellulosic fibers are generally conveyed in a gaseous stream (or by mechanical means) rather than a liquid stream. For example, the cellulosic fibers may be first coated with a thermosetting resin binder, such as a phenol-formaldehyde resin, and are then randomly distributed into a mat by air blowing the resin-coated fibers onto a support member. The mat, typically having a moisture content of less than 30 wt. % and preferably less than 10 wt. %, is then pressed under heat and pressure to cure the thermosetting resin and to compress the mat into an integral consolidated structure.
A wet-dry forming process may also be used to produce wood composites. Generally, in a wet-dry process, a slurry is formed of water, a cellulosic filler, and a resin binder. Sufficient water is then drained from the slurry to form a wet mat. Further water is then removed from the wet mat by evaporation, which is preferably facilitated by the application of heat, in order to form a dried mat. The dried mat is then pressed under heat to form the wood composite.
Preferably, a wet-dry process begins by blending cellulosic or wood fiber raw material in a vessel with large amounts of water having a pH of less than 7 to form a slurry. This slurry is then blended with the resin binder. The blend is then deposited onto a water-pervious support member, where a large percentage (e.g. 50%) of the water is removed, thereby leaving a wet mat of cellulosic material. Further water may be removed in a second step, in which case these two steps may be referred to respectively as the primary water removal step and the secondary water removal step. This wet mat is then transferred to an evaporation zone where much of the remaining water is removed by evaporation, for example by heating the wet mat. The mat may be further dried in a second evaporation step, in which case these two evaporation steps may be referred to respectively as the primary evaporation step and the secondary evaporation step. (These steps are commonly referred to as "drying" steps.) The dried mat preferably has a moisture content of less than about 10 wt. %. The dried mat is then transferred to a press and consolidated under heat and pressure to form the wood composite which may be, for example, a flat board or in any desired shape depending on its intended use.
Wood composites produced according to the processes described above may, however, have poor surface quality. Poor surface quality is indicated by a wood composite having a porous or open surface, inadequate consolidation along the edges or corners of the wood composite and/or poor definition of wood grain which is often embossed on the surface of the wood composite. Poor surface quality is also indicated where the wood composite exhibits poor internal bonding and strength, especially at the edges of the wood composite. For example, poor surface quality is shown where there appears to be layers within the composite which are "flaky" and can be easily peeled away; this phenomenon may be referred to as a lack of surface tightness. Where there is poor surface quality, the wood composite may easily break apart and the product is therefore unsatisfactory.
In order to help prevent the problems associated with poor surface quality, a urea solution has been used as a surface treatment. The urea converts to ammonia under heat, which then plasticizes the fibers during consolidation. However, the use of urea has several disadvantages, including the relatively high application rate which is required (up to two grams of urea solids per square foot), contribution to the build up of undesirable material (e.g., a film which includes carbon and other materials, commonly referred to as a "carbon film") on the die surfaces, promotion of corrosion on unplated die surfaces, and the addition of ammonia to the press exhaust stream. The carbon film may damage the final product and/or the die surfaces and is difficult to remove from the die surfaces. The presence of ammonia in the exhaust gases is of particular concern if thermal oxidation is used as a pollution control measure, because the ammonia can convert to oxides of nitrogen (NO.sub.x), which are hazardous. It is therefore desirable to reduce or eliminate the need for a urea treatment during pressing of the wood product.
The surface of the wood composite may also be improved by postpress tempering with drying oils. Many different types of tempering oils may be used, including linseed oil, soybean oil, tung oil, oiticica oil, and unsaturated fatty acid esters. However, the use of these oils increases both cost and production time. In addition, use of such oils may be environmentally undesirable. It is therefore desirable to reduce or eliminate the need for such postpress treatment of the molded wood product.